Problem: newer cmake versions set CMAKE_C_FLAGS at a later stage not allowing a user to override the standard

*Please use this template for reporting suspected bugs or requests for help.*



# Issue description

When trying to override the default C/CXX standard CMAKE_CXX_FLAGS and CMAKE_C_FLAGS are used. In modern cmake it is customary to use CMAKE_CXX_STANDARD and CMAKE_C_STANDARD and only set the CMAKE_CXX_FLAGS and CMAKE_C_FLAGS per target.





# Environment



* libzmq version (commit hash if unreleased): 519e2bf1291829da41a108189f6822e9446062d9

* OS: windows 10



# Minimal test code / Steps to reproduce the issue



build libzmq with -DCMAKE_CXX_STANDARD=17 and observe the project is still built with c++11.



# What's the actual result? (include assertion message & call stack if applicable)

check_c(xx)_compiler flag is still called and CMAKE_C_FLAGS and CMAKE_CXX_FLAGS are still appended with -std=xxx.



# What's the expected result?

Not overriden cppstd

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